Here are three excellent resources for studying your Bible. All three are available in print as Study Bibles and are worth owning in that format. But the great news is they are available for free online. (ESV Study Bible is only free this month.)
The ESV Study Bible Online (free in November, normally costs $22.50)
The ESV Study Bible is the king of study Bibles. The book introductions and articles on various topics are a tremendous resource, and that’s all in addition to the textual notes you normally think of as the meat of a study Bible. Jump on the opportunity to get this resource for free. You’ll create an account at esvbible.org and then add the web app to your account.
The NET Bible (always free)
The NET Bible is a translation and study Bible together published by scholars at Dallas Theological Seminary. The notes here are in-depth and often more technical than what you find in general study Bibles. They often deal more with translation and interpretive questions than with the theology of the text. I don’t know of another free resource where you can get this kind of quality information. And if your question touches on some of these textual and translation areas, other study Bibles may not even address the issues.
The Holman Christian Standard Study Bible (always free)
The HCSB Study Bible is a quality general study Bible. The website has a number of other free tools integrated with the study Bible as well as modules you can add by purchasing. I don’t think the interface is as polished as the other two and the resource itself is a solid second choice for general study Bible behind the excellent ESV Study Bible.